1. Field of the Invention
The present invention relates to variable mechanical advantage first class levers, such as shift levers, and in particular to variable mechanical advantage shift levers for selectively shifting synchronized positive clutch type mechanical change gear transmissions.
2. Description of the Prior Art
Shift levers for shifting sliding clutches and/or sliding gears into and out of driving engagement are well know in the prior art. Shift levers for shifting mechanical transmisisons are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,259,877; 3,915,027 and 3,850,047, all of which are hereby incorporated by reference. Typically, such shift levers are first class levers pivotably mounted in a tower assembly and include a first end portion usually gripped and pivotably moved by a driver/operator and a second end portion extending into and cooperating with a shift bar housing assembly having a plurality of axially movable shift rails each carrying a shift fork and a shift block member thereon. Typically, the shift lever is pivoted in one direction to engage the second portion thereof with the shift block member carried by a selected shift rail and is then pivoted in a transverse direction to axially move the shift rail and shift fork carried thereby.
Shift levers of this type are first class levers defined by a first portion usually gripped by the operator, a second portion usually engageable with one or more shift rails and a pivot point located between the first and second portions. As is known, if the first portion is of a fixed length, the mechanical advantage of the shift lever will vary inversely with the effective length of the second portion and the required travel of the first portion to achieve a given axially movement of a shift rail will vary directly with the effective length of the second portion. In prior art shift levers of this type, the effective length of the second portion is usually fixed and is a compromise between desired mechanical advantage and a desire to minimize the required pivotal movement of the first portion to achieve a given movement of the engaged shift rail. Shift levers of this type may be seen by reference to U.S. Pat. No. 3,934,485, hereby incorporated by reference.
Synchronized mechanical transmissions are well known in the prior art and may be seen by reference to U.S. Pat. Nos. 4,307,624; 3,929,029 and 3,221,851, all hereby incorporated by reference. Typically, such transmissions comprise constantly meshed gears on parallel shafts with synchronized positive clutches to selectively clutch a selected one of the gears for rotation with one of the shafts. Such synchronized positive clutches typically comprise an axially fixed positive clutch member (usually fixed to a gear) and an axially slideable positive clutch member (usually splined to a shaft for rotation therewith and axial movement relative thereto) movable towards and away from the fixed clutch member. The axially slideable positive clutch member is axially mounted on the shaft and movable toward and away from the fixed positive clutch member by a shift fork or the like. A blocking mechanism is provided interposed the two positive clutch members to prevent engagement of the positive clutch members if they are not rotating at a synchronous or substantially synchronous speed. A relatively high torque capacity friction synchronizer clutch is provided for causing the two positive clutch members to rotate at a synchronous speed, such friction synchronizer clutch being applied by the axially movable positive clutch member usually through the blocker means. When the positive clutch members are caused to rotate at a substantially synchronous speed, the blocker mechanism will "37 unblock" allowing the axially movable positive clutch member to move axially therethrough and into positive engagement with the axially fixed positive clutch member. In transmissions carrying relatively large torque loads, such as mechanical change gear transmissions for heavy duty trucks, the axial force required to properly engage the frictional synchronizing clutches is considerable.
The axial movement of the axially movable positive clutch member from a fully disengaged to a fully engaged position may be separated into three distinct segments. The first segment is an initial movement of the axially movable positive clutch member into engagement with the blocker and initial engagement of the frictional synchronizing clutch. This first segment of axial movement typically requires a relatively small axial movement and a relatively low axial force. The second segment is axial movement to fully frictionally engage the synchronizer clutch sufficiently to cause synchronization of the positive clutch members. This second segment of clutch movement requires very little axial movement (compression of the friction surfaces) and a relatively high axial force. The third segment is a final clutch movement after synchronization is achieved and the unblocking means has unblocked wherein the axially movable clutch member moves through the blocker and into engagement with the other positive clutch member. This final segment of axial movement typically requires a relatively low axial force and a relatively large axial travel.
The prior art shift levers for synchronized mechanical transmissions for heavy duty vehicles were not totally satisfactory as if the second lever portion was of a short enough effective length to provide a satisfactory mechanical advantage, the required travel of the first portion was larger than desired and often objectionable, especially in the cab of a heavy duty truck where space is often very limited and if the second portion was of a large enough effective length to provide an acceptable travel of the first portion the mechanical advantage thereof was reduced requiring an often objectionable amount of driver effort to engage the frictional synchronizing clutches.